Method of Protecting a Component Against Hot Corrosion

ABSTRACT

A method of protecting a component against hot corrosion comprising the steps: (a) applying a chromium diffusion coating to the component; and (b) applying a further coating to selected regions of the chromium diffusion coating, the selected regions being chosen dependent on subsequent use of the component.

This invention relates to a method of protecting a component against hotcorrosion.

The invention finds particular application in the protection against hotcorrosion of a gas turbine engine rotor blade.

It is known that chromium provides excellent protection against socalled Type I and Type II hot corrosion. In this regard, diffusioncoatings produced by the diffusion of chromium and aluminium into thealloy substrate have long been used to provide this protection. MCrAlYoverlay coatings (where M is Ni or Co or a combination of the two) havebeen used as an alternative to diffusion coatings at higher temperaturesto protect against oxidation. The use of diffused chromium alone(chromising) is known to provide excellent protection against relativelylow temperature Type II hot corrosion, and further to be strain tolerant(to have no effect on the fatigue properties of the substrate).

According to the present invention there is provided a method ofprotecting a component against hot corrosion comprising the steps: (a)applying a chromium diffusion coating to the component; and (b) applyinga further coating to selected regions of the chromium diffusion coating,the selected regions being chosen dependent on subsequent use of thecomponent.

Preferably, the selected regions are regions not subject to higherphysical stress in the subsequent use of the component.

In a first method according to the present invention described below,the further coating comprises an aluminium diffusion coating.

In a second method according to the present invention described below,the further coating comprises an MCrAlY overlay coating, where M is Nior Co or a combination of the two.

In a third method according to the present invention described below,the further coating comprises an MCrAlY overlay coating, where M is Nior Co or a combination of the two, and the method further comprises thestep (c) applying an aluminium diffusion coating to the selected regionscoated with the MCrAlY overlay coating.

In the second and third methods the MCrAlY overlay coating applied instep (b) suitably comprises: 30 to 70 weight % Nickel; 30 to 50 weight %Cobalt; 15 to 25 weight % Chromium; 5 to 15 weight % Aluminium; and upto 1 weight % Yttrium.

In the second and third methods the MCrAlY overlay coating applied instep (b) may additionally include one or more elements selected from thegroup consisting of rhenium, silicon and hafnium.

The chromium diffusion coating applied in step (a) suitably comprises 15to 30 weight % chromium and is 5 to 25 microns thick.

Methods according to the present invention find particular applicationin the protection against hot corrosion of nickel based superalloycomponents.

Methods according to the present invention find particular applicationin the protection against hot corrosion of gas turbine engine rotorblades.

The present invention also extends to components protected against hotcorrosion by means of a method according to the present invention.

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which:

FIG. 1 shows a gas turbine engine rotor blade and the coating of thisblade using a first method in accordance with the present invention;

FIG. 2 is a view of a side of the rotor blade of FIG. 1 hidden in FIG. 1but to be seen when looking from the right in FIG. 1;

FIG. 3 shows a gas turbine engine rotor blade and the coating of thisblade using a second method in accordance with the present invention;

FIG. 4 is a view of a side of the rotor blade of FIG. 3 hidden in FIG. 3but to be seen when looking from the right in FIG. 3;

FIG. 5 shows a gas turbine engine rotor blade and the coating of thisblade using a third method in accordance with the present invention; and

FIG. 6 is a view of a side of the rotor blade of FIG. 5 hidden in FIG. 5but to be seen when looking from the right in FIG. 5.

In each of the first to third methods the rotor blade coated is a nickelbased superalloy rotor blade. The rotor blade may be produced byconventional or directionally solidified (including single crystal)casting techniques. Typical alloys are MarM247, IN6203 and CMSX-4.

Referring to FIGS. 1 and 2, the blade coated comprises an outer shroudpart 1 (above dotted line A), an aerofoil part 3 (between dotted lines Aand B), a platform part 5 (between dotted lines B and C), and a rootpart 7 (below dotted line C). The blade includes an internal coolingpassage 9 which commences as shown in FIG. 1, loops within the blade,and exits (exit not shown) via the top side of shroud part 1.

In a first stage of the first method, all surfaces of all parts of theblade, including internal cooling passage 9, are chromised, i.e.chromium is diffused into the surfaces. This diffusion is achieved byany suitable method, e.g. pack cementation or chemical vapour deposition(CVD). This results in a surface layer rich in chromium. The layershould typically contain 15 to 30 weight % chromium and be 5 to 25microns thick.

In a second stage of the first method, an aluminium diffusion coating isapplied to all external surfaces of the blade above dotted line M. Thisdiffusion is again achieved by any suitable method, e.g. packcementation or CVD. Masking is employed below dotted line M to preventstray aluminium depositing below this line. If such stray depositingdoes occur, this is acceptable between dotted lines M and S, but notbelow dotted line S, i.e. not on the so called fir tree root of rootpart 7. Thus, a chromium modified aluminide coating results on allexternal surfaces of the blade above dotted line M. The so called outerbeta layer of the chromium modified aluminide coating should typicallycontain 15 to 30 weight % aluminium and 5 to 15 weight % chromium. Thetotal thickness of the chromium modified aluminide coating, includinginter-diffusion zone, should typically be 25 to 100 microns.

Finally, the blade is heat treated to ensure that it maintains itsoptimum mechanical properties.

It is to be noted that in the final blade, all external surfaces abovedotted line M are chromised plus aluminised, whereas all externalsurfaces below dotted line M and internal cooling passage 9 arechromised only.

Referring to FIGS. 3 and 4, the blade coated comprises an outer shroudpart 1 (above dotted line A), an aerofoil part 3 (between dotted lines Aand B), a platform part 5 (between dotted lines B and C), and a rootpart 7 (below dotted line C). The blade includes an internal coolingpassage 9 which commences as shown in FIG. 3, loops within the blade,and exits (exit not shown) via the top side of shroud part 1.

In a first stage of the second method, all surfaces of all parts of theblade, including internal cooling passage 9, are chromised, i.e.chromium is diffused into the surfaces. This diffusion is achieved byany suitable method, e.g. pack cementation or CVD. This results in asurface layer rich in chromium. The layer should typically contain 15 to30 weight % chromium and be 5 to 25 microns thick.

In a second stage of the second method, an MCrAlY overlay coating (whereM is Ni or Co or a combination of the two) is applied to the followingparts of the blade: outer shroud part 1, aerofoil part 3, and the topface 11 of platform part 5. The coating suitably comprises 30 to 70weight % Nickel, 30 to 50 weight % Cobalt, 15 to 25 weight % Chromium, 5to 15 weight % Aluminium, and up to 1 weight % Yttrium. The coating mayadditionally include one or more elements selected from the groupconsisting of rhenium, silicon and hafnium. The coating is applied byany suitable method, e.g. by thermal spray techniques such as vacuumplasma spraying (VPS), low pressure plasma spraying (LPPS), and highvelocity ox-fuel spraying (HVOF), or by electroplating. Masking isemployed to ensure that MCrAlY is not deposited on the remainder ofplatform part 5 below top face 11, and on root part 7.

Finally, the blade is heat treated to ensure that it maintains itsoptimum mechanical properties.

It is to be noted that in the final blade, outer shroud part 1, aerofoilpart 3, and the top face 11 of platform part 5 are chromised plus havean overlay coating of MCrAlY, whereas the remainder of platform part 5below top face 11, root part 7, and internal cooling passage 9 arechromised only.

Referring to FIGS. 5 and 6, the blade coated comprises an outer shroudpart 1 (above dotted line A), an aerofoil part 3 (between dotted lines Aand B), a platform part 5 (between dotted lines B and C), and a rootpart 7 (below dotted line C). The blade includes an internal coolingpassage 9 which commences as shown in FIG. 5, loops within the blade,and exits (exit not shown) via the top side of shroud part 1.

In a first stage of the third method, all surfaces of all parts of theblade, including internal cooling passage 9, are chromised, i.e.chromium is diffused into the surfaces. This diffusion is achieved byany suitable method, e.g. pack cementation or CVD. This results in asurface layer rich in chromium. The layer should typically contain 15 to30 weight % chromium and be 5 to 25 microns thick.

In a second stage of the third method, an MCrAlY overlay coating (whereM is Ni or Co or a combination of the two) is applied to the followingparts of the blade: outer shroud part 1, aerofoil part 3, and the topface 11 of platform part 5. The coating suitably comprises 30 to 70weight % Nickel, 30 to 50 weight % Cobalt, 15 to 25 weight % Chromium, 5to 15 weight % Aluminium, and up to 1 weight % Yttrium. The coating mayadditionally include one or more elements selected from the groupconsisting of rhenium, silicon and hafnium. The coating is applied byany suitable method, e.g. by thermal spray techniques such as VPS, LPPS,and HVOF, or by electroplating. Masking is employed to ensure thatMCrAlY is not deposited on the remainder of platform part 5 below topface 11, and on root part 7.

In a third stage of the third method, those parts of the blade to whichthe MCrAlY overlay coating was applied (outer shroud part 1, aerofoilpart 3, and the top face 11 of platform part 5) are over-aluminised,i.e. an aluminium diffusion coating is applied to these parts. Thediffusion is achieved by any suitable method, e.g. pack cementation orCVD. Masking is employed to ensure that stray aluminium is not depositedon the remainder of platform part 5 below top face 11, and on root part7. The result of the over-aluminisation should be that the outer surfaceof the MCrAlY overlay coating has an aluminium content of typically 15to 30 weight %. The total thickness of the over-aluminised MCrAlYcoating, including inter-diffusion zone, should typically be 100 to 200microns.

Finally, the blade is heat treated to ensure that it maintains itsoptimum mechanical properties.

It is to be noted that in the final blade, outer shroud part 1, aerofoilpart 3, and the top face 11 of platform part 5 are chromised plus havean overlay coating of MCrAlY, which MCrAlY overlay coating has beenover-aluminised, whereas the remainder of platform part 5 below top face11, root part 7, and internal cooling passage 9 are chromised only.

It is to be appreciated that in the above described first to thirdmethods the application of further coating(s) in addition to the initialchromium diffusion coating is restricted to regions of the rotor bladenot subject to higher physical stress in use of the blade. In the firstmethod, the diffused aluminium coating is restricted to all externalsurfaces above dotted line M. In the second method, the MCrAlY overlaycoating is restricted to outer shroud part 1, aerofoil part 3, and thetop face 11 of platform part 5. In the third method, the MCrAlY overlaycoating plus over-aluminisation is restricted to outer shroud part 1,aerofoil part 3, and the top face 11 of platform part 5. Thisrestriction ensures that those regions of the blade that are subject tohigher physical stress are coated with a chromium diffusion coatingalone which is strain tolerant, and that the strain tolerance of thiscoating is not degraded by the application of further coating(s). Thepurpose of the application of the further coating(s) is to provideadditional protection against hot corrosion. The approach takentherefore with regard to the application of the further coating(s) is asfollows. It is first determined where on the blade there will be aregion of relatively high temperature. Further coating(s) are thenapplied to this region provided it is not also a region that willexperience higher physical stress.

1-20. (canceled)
 21. A turbine blade comprising: an aerofoil; a shroudarranged at a first end of the aerofoil; a platform arranged at a secondend of the aerofoil; a root arranged adjacent the platform; and aprotective coating system for protecting the turbine blade against hotcorrosion, wherein the coating system has: a first layer with a chromiumdiffusion coating applied to the turbine blade, and a second layerarranged on top of the first layer having an aluminium diffusion coatingapplied to the aerofoil, the shroud, the platform and the root, whereinthe second layer covers the root only in part.
 22. The turbine bladeaccording to claim 21, wherein the first layer covers the entire turbineblade.
 23. The turbine blade according to claim 21, wherein the chromiumdiffusion coating comprises 15 to 30 weight % chromium and is 5 to 25microns thick.
 24. The turbine blade according to claim 21, wherein theturbine blade is a nickel based superalloy turbine blade.
 25. A turbineblade comprising: an aerofoil; a shroud arranged at a first end of theaerofoil; a platform arranged at a second end of the aerofoil, theplatform comprising a top face arranged adjacent the aerofoil; aprotective coating system for protecting the turbine blade against hotcorrosion, wherein the coating system comprises: a first layer, thefirst layer comprising a chromium diffusion coating applied to theturbine blade, and a second layer, the second layer arranged on top ofthe first layer and comprising a MCrAlY overlay coating, wherein M isselected from the group consisting of Ni, Co and a combination of Ni andCo, and the overlay coating is applied only to the aerofoil, the shroudand the top face of the platform.
 26. The turbine blade according toclaim 25, wherein the first layer covers the entire turbine blade.
 27. Aturbine blade according to claim 25, wherein the turbine blade comprisesa third layer, wherein the third layer is an aluminium diffusioncoating, arranged on top of the second layer.
 28. The turbine bladeaccording to claim 25, wherein the MCrAlY overlay coating comprises: 30to 70 weight % Nickel, 30 to 50 weight % Cobalt, 15 to 25 weight %Chromium, 5 to 15 weight % Aluminium, and up to 1 weight % Yttrium. 29.The turbine blade according to claim 25, wherein the MCrAlY overlaycoating additionally includes at least one element selected from thegroup consisting of rhenium, silicon and hafnium.
 30. The turbine bladeaccording to claim 25, wherein the chromium diffusion coating comprises15 to 30 weight % chromium and is 5 to 25 microns thick.
 31. The turbineblade according to claim 25, wherein the turbine blade is a nickel basedsuperalloy turbine blade.
 32. A method of protecting a turbine bladeagainst hot corrosion, comprising: providing the turbine bladecomprising: an aerofoil, a shroud arranged at a first end of theaerofoil, a platform arranged at a second end of the aerofoil, and aroot arranged adjacent the platform; applying a first layer comprising achromium diffusion coating to the turbine blade; and applying a secondlayer comprising an aluminium diffusion coating to the aerofoil, theshroud, the platform and the root, wherein the second layer covers theroot only in part.
 33. The method according to claim 32, wherein thechromium diffusion coating comprises 15 to 30 weight % chromium and is 5to 25 microns thick.
 34. The method according to claim 32, wherein theturbine blade is a nickel based superalloy turbine blade.
 35. A methodof protecting a turbine blade against hot corrosion, comprising:providing the turbine blade, wherein the turbine blade comprises: anaerofoil, a shroud arranged at a first end of the aerofoil, and aplatform arranged at a second end of the aerofoil, the platformcomprising a top face arranged adjacent the aerofoil; applying a firstcoating layer including a chromium diffusion coating to the turbineblade; and applying a second coating layer on top of the first coatinglayer to the aerofoil, the shroud and the top face of the platform,wherein the second layer is an MCrAlY overlay coating, and wherein M isselected from the group consisting of Ni, Co and a combination of Ni andCo.
 36. The method according to claim 35, further comprising applying athird coating layer on top of the MCrAlY coating, wherein the thirdcoating is an aluminium diffusion coating.
 37. The method according toclaim 35, wherein the MCrAlY coating comprises: 30 to 70 weight %Nickel; 30 to 50 weight % Cobalt; 15 to 25 weight % Chromium; 5 to 15weight % Aluminium; and up to 1 weight % Yttrium.
 38. The methodaccording to claim 35, wherein the MCrAlY overlay coating includes atleast one element selected from the group consisting of rhenium, siliconand hafnium.
 39. A method according to claim 35, wherein the chromiumdiffusion coating comprises 15 to 30 weight % chromium and is 5 to 25microns thick.
 40. A method according to claim 35, wherein the turbineblade is a nickel based superalloy turbine blade.